Utilising Industrial Wastewater With Zero Liquid Discharge (ZLD)
What is wastewater?
Wastewater is water that has been used either in homes, in business, in agriculture, or as part of an industrial process. Simply put, it is used water.
More often than not this wastewater or ‘used water’ will contain physical, chemical, or biological contaminants picked up along the way which can result in the water being deemed unsafe for reuse.
Without appropriate waste management the wastewater is discharged either directly or indirectly (to remove harmful contaminants) back into bodies of water around the globe where it is no longer considered a fresh water supply.
FACT: Despite the fact that water comprises more than 70% of the earth’s surface, only around 2.5% of this water can be qualified as “fresh” and less than 1% of this is accessible.
At the moment 80% of wastewater is returned to our oceans, rivers and lakes - most of this has been left untreated.
The issue is stated right there in the name - WASTE - water.
With such limited supplies of freshwater, the water we do use should be utilised to its fullest.
For the industrial sector without utilising as much of it as possible (recycle and reuse) before disposing of it appropriately is simply cutting into profits across the board.
In response companies are finding new ways to optimise their wastewater outputs through a process called Zero Liquid Discharge.
What Is Industrial Wastewater?
Industrial industries generate huge volumes of wastewater on a daily basis.
Processes like cooling tower blowdown, ion exchange regenerative streams, wet flue gas desulfurization systems etc. are all extremely water intensive.
The primary industries that consume the largest amounts of water are pulp and paper, mining, oil & gas, iron & steel, food, and chemical.
Industrial processes face a three-pronged assault:
- They require huge amounts of fresh water and therefore water availability continues to be a concerning factor.
- The vast amounts of wastewater returned from these industrial processes have not been utilised fully which will see profits fall
- The growing public unease along with the continued scarcity of available water, has caused governments in several countries to impose strict regulations regarding wastewater management and disposal. Including changes within the industry such as rising expenses for wastewater disposal, and increased value of freshwater
According to eurostat up to 40% of industrial wastewater does not receive any treatment before being disposed.
These challenges for the industrial sectors have given rise to innovative procedures that aims to tackle all three; water scarcity, water utilisation, and water regulations.
One such wastewater management strategy is Zero Liquid Discharge (ZLD).
What Is Zero Liquid Discharge (ZLD)?
ZLD is an ambitious strategy that uses advanced wastewater treatment technologies to ensure all water is recovered and contaminants are reduced to a solid waste.
The three main objectives for a Zero Liquid Discharge system are:
- Eliminate liquid wastewater discharge to zero
- Generate solids for landfill disposal or reuse
- Purify and recycle high-quality water for reuse
The benefits are clear:
- The water is utilised to its fullest resulting in less being required, saving water attainment costs and reserving freshwater availability
- Plants can treat and recover other potentially valuable resources from waste streams which can then be refined and used in other processes or sold as co-products from the original process.
- It supports plants with meeting their discharge and water reuse requirements imposed by government entities.
- It avoids negative environmental impacts of wastewater discharge and reduces the corresponding public concerns demonstrating corporate responsibility.
Where industrial users see cause for concern in adopting this technology is very simply costs.
As the industrial sector is being further and further backed into a corner by rising environmental concerns, stringent regulations, and increased costs for wastewater disposal and freshwater attainment, ZLD is seen as an option that industrials might need to take if and when their hand is forced - but it comes at a cost.
The process requires implementing new equipment, personnel, and procedures - but the process is now established and Howden can ease the implementation hurdles.
How Does Zero Liquid Discharge (ZLD) Work?
With a basic understanding of the reasoning behind the adoption of Zero Liquid Discharge we can now delve into how this system works.
The exact components of a ZLD treatment system varies and will depend on:
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The volume of dissolved, undissolved, and organic material present in the waste
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The system’s required flow rate
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What specific contaminants are present
But in general, early ZLD systems were based on thermal processes that takes the wastewater (1), evaporates it in a brine concentrator (2) followed by a brine crystallizer (3) and then the condensed distillate water is collected for reuse, while the produced solids are sent to a landfill for disposal or recovered as byproducts (4).
ZLD Step-By-Step
1. The feed wastewater undergoes some form of pre-treatment including filtration, pH adjustments, deaeration, antiscalant and other physicochemical and biological treatments. This removes and reduces materials that would scale or foul the following treatment steps such as metals and silica. However, these pre-treatment methods mostly involve intensive use of chemicals, producing additional solid waste and increasing operating costs.
2. After pre-treating the waste stream the next step is to use thermal processes or evaporation within the brine concentrator to collect recovered evaporated water for reuse while the brine is concentrated to a higher solids concentration up to the initial crystallization point.
3. The exiting condensed distillate brine from the evaporator goes to the crystallizer where the water is concentrated further until the impurities crystallize. The resulting product is dewatered by a filter press or a centrifuge and the remaining water is further recovered for reuse.
4. The collected water from the three steps above is returned to the process - eliminating any liquid discharge from the system. The solid waste is either sent to a landfill for disposal or recovered as valuable salt byproducts for sale or reuse in other processes.
To improve energy efficiency, membrane-based salt-concentrating technologies have been introduced into ZLD systems to reduce the volume of concentrated brine entering the brine concentrators and crystallizers (which require vast amounts of energy).
These membrane-based technologies pre-concentrate the effluent wastewater stream to a high salinity and are able to recover up to 80% of the water. These technologies include:
Reverse osmosis (RO)
Concentration in the earlier stages of ZLD is usually done with membranes like reverse osmosis (RO), a hydraulic pressure-driven desalination technology that requires low energy in comparison to thermal processes.
RO captures the majority of dissolved solids that flow through the process, however, RO requires extensive pre-treatment such as chemical softening, ion exchange, ultrafiltration, and pH adjustments to prevent membrane fouling or scaling which reduces water permeability and the lifespan of RO membranes.
However, RO, although much more energy efficient than thermal evaporation, cannot operate at very high hydraulic pressure which means RO can only be applied to feedwaters with a limited salinity range.
New technologies have emerged that tolerate higher salinities than RO and consume less energy than brine concentrators. These technologies are electrodialysis (ED), forward osmosis (FO), and membrane distillation (MD). They are often used alongside RO to provide very high water recovery and concentration before being fed to the crystallizer.
Electrodialysis (ED)
Electrodialysis (ED) is a membrane process that creates an electric field which pushes negative and positive ions through semipermeable membranes, selectively allowing the transport of counterions and preventing the passage of co-ions.
Used in stages, ED concentrates the brine and generates two streams:
- Salt-depleted dilute
- Concentrated brine
A modified form of ED, electrodialysis reversal (EDR) reverses the polarity of the electrodes frequently which minimises fouling and scaling. Thereby less pre-treatment is needed in comparison to RO.
ED and EDR can concentrate feed waters to a higher salinity, however, they have a low scaling propensity for silica. Therefore ED/EDR is not capable of producing usable water from this process alone which is one of the key requirements from a ZLD system.
ED/EDR has often been applied in combination with RO to achieve higher salinity limits while reducing energy consumption relative to brine concentrators.
Forward osmosis (FO)
Forward osmosis (FO) uses osmotic pressure difference to prompt water permeation across a semipermeable membrane. The water flows from the feedwater to a concentrated draw solution that has a higher osmotic pressure.
The produced brine is sent on to the crystallizer, and the draw solutes are separated from the desalinated water to regenerate the concentrated draw solution - therefore being reused.
So unlike RO, FO does not use applied pressure in order to achieve separation of water from dissolved ions. This means that a lot less energy is required, and it can treat waters with much higher salinity than RO. Due to working at low-pressure FO also has a much lower fouling propensity than RO.
FO can also be used as a brine concentrator after the RO stage.
Membrane Distillation
Membrane distillation (MD) is a thermal, membrane-based, desalination process that uses hydrophobic membranes.
The feedwater is heated to typically 60−90 °C on one side, this temperature difference between the hot feedwater and the colder permeate side creates a vapor pressure difference that drives the water vapor across the hydrophobic, microporous membranes.
The water vapor can then be collected via direct contact membrane distillation (DCMD) or collected on a condensation surface separated from the membrane.
MD is capable of treating high salinity feed waters but is more energy-intensive than both RO and ED/EDR. However, MD can use waste heat and/or alternative energy sources to achieve cost savings. It also has low fouling propensity.
MD can be combined with other processes in integrated systems.
Drawbacks of Zero Liquid Discharge (ZLD)
The main goal of ZLD is to reduce all water waste and increase the amount of water reuse within an industrial process plant.
Despite this, the process of Zero Liquid Discharge does result in its own negative outcomes.
ZLD genrates solid wastes which then have their own treatments or disposal challenges and costs.
The other, more prominent drawback from the industry side is that of costs - ZLD demands huge investments in equipment, time, personnel, and maintenance.
High cost and intensive energy consumption will remain the main barriers to ZLD adoption.
The Future of Zero Liquid Discharge
ZLD as a wastewater management strategy is growing due to the severe environmental regulations and costs that are being placed on the industrial industries.
With advances in improving the energy and cost efficiencies ZLD can become much more feasible and sustainable in the future.
In terms of lowering GHG, ZLD can incorporate technologies such as MVR, RO, FO, ED, and MD which are much more energy efficient but much more costly.
Certain ZLD technologies can even make use of low-grade renewable energy such as waste heat and geothermal energy. Resource recovery may provide an additional economic incentive for ZLD.
Future growth of the ZLD market will heavily rely on regulatory incentives that outweigh its economic disadvantages.
Howden supplies blowers and fans that are integral to Zero Liquid Discharge systems based on the MVR process. Find out more about Mechanical Vapour Recompression.
Howden's range of compressors and fans perform critical functions in the water industry. If you want to know more about wastewater treatment - Access our wastewater documentation.